The long-term objective of this research is to characterize the roles of flavin-containing monooxygenases (FMOs) in the metabolism and toxicity of various drugs, toxicants, endogenous compounds, and their major metabolites. The FMO family is comprised of at least five isoforms, which catalyze oxidation of compounds containing N-, S-, and Se-atoms. With most chemicals, the FMO-mediated reaction is a detoxication reaction. However, with compounds such as selenomethionine, a naturally occurring compound that has antioxidant and anticancer properties, and the cysteine S-conjugates of the environmental contaminants trichloroethylene and tetrachloroethylene, the FMO-mediated pathway is a bioactivation reaction. Another known FMO substrate, methionine, has been implicated in toxicity in humans with inborn errors of homocysteine metabolism (homocystinuria) and in infants receiving total parenteral nutrition. Additionally, genetic defects in FMO3 have been associated with the inability to metabolize the dietary constituent trimethylamine, a situation that causes "fish-odor syndrome". Thus, examination of FMO expression in animal and human tissues, the substrate specificities of the various FMOs, and the biochemical and toxic effects of the FMO-derived metabolites will allow for a more accurate, mechanism-based assessment of risk associated with human exposure to these chemicals. It will also guide the search for safer drugs and chemicals. In this application, the hypothesis that FMOs play important roles in metabolism and toxicity of cysteine S-conjugates, selenomethionine, and methionine will be examined. Experiments using both rodent and human cells and/or tissues in vitro and in rodents in vivo will be carried out. We will develop immunochemical and functional probes to provide further characterization of rodent and human FMOs. A major focus of this project is on FMO4, whose expression has not been clearly characterized at the native protein level in any species. The specific aims are to: 1) develop immunochemical and 2D gel electrophoretic methods to characterize rat and human FMOs; 2) purify and characterize FMO4 from rat kidney; 3) characterize the roles of FMOs in metabolism of potential substrates; 4) characterize the role of FMOs in the metabolism and toxicity of methionine and selenomethionine; and 5) further investigate the role of FMOs in the metabolism and toxicity of the cysteine S-conjugate of trichloroethylene.